Reducing the electro-optic threshold of a liquid crystal device facilitates a reduction in the drive voltage required to switch the cell between optical states. For example, the electro-optic threshold of a standard twisted nematic device is about 2.0 volts. This threshold voltage is characteristic of the liquid crystal material contained in the cell.
In the past, two approaches have been taken to reduce the drive voltages of standard twisted nematic cells. One approach entails the use of high dielectric constant liquid crystal material with low elastic constants, and the other entails the use of a high pretilt alignment. FIG. 1 shows the electro-optic threshold curves for six standard twisted nematic cells fabricated with combinations of liquid crystal material and alignment techniques. (The liquid crystal materials identified are manufactured by Merck.) As an example, curves 1 and 2 represent cells filled with the same liquid crystal material, curve 1 indicating the performance with a high pretilt silicon monoxide (SiO) alignment and curve 2 indicating the performance with a low pretilt polyvinyl alcohol (PVA) alignment. The higher pretilt results in about a 0.6 volt lower electro-optic threshold.
Neither of these approaches is desirable for the following reasons. The high dielectric constant material introduces higher ion solubility that manifests itself as an image sticking effect in active matrix addressed displays. The higher pretilt alignment is achieved by evaporated alignment, which is neither an easy nor an economical process.
What is needed, therefore, is a liquid crystal cell that can operate in an image display system as a switch that changes optical states in response to relatively low applied drive voltages.